In situ transformations of Pd/NHC complexes with N-heterocyclic carbene ligands of different nature into colloidal Pd nanoparticles

R-NHC coupling was previously considered as a process of degradation of M/NHC species; however recent studies have pointed out that it may be responsible for the generation of catalytically active NHC-free complexes and/or metallic nanoparticles. Therefore, a detailed and systematic study of R-NHC coupling for various carbene ligands is an important topic. In the present article this process has been studied for reactive aryl iodide coupling partners by a combination of quantum chemical calculations and continuous reaction monitoring via pressurized sample infusion electrospray ionization mass spectrometry (PSI-ESI-MS). DFT calculations revealed a strong tendency of (NHC)Pd(Ph)(I)DMF complexes bearing various N-heterocyclic carbene (NHC) ligands to undergo Ph-NHC coupling. Calculated energy barriers of these reactions lie in the range of 17.9-25.1 kcal mol(-1). Ph-NHC coupling is thermodynamically more favorable for the complexes containing unsaturated NHC ligands with bulky substituents. NBO analysis suggests that the process of Ph-NHC formation is similar for different NHC ligands. In order to confirm the results of theoretical studies, a series of ESI-MS reaction monitoring experiments were performed for (NHC)Pd(i)(2)(Py) and (NHC)Pd(Cl)((3)-1-Ph-C3H4) complexes interacting with iodobenzene, where Ph-NHC coupling products were observed in all cases. As direct experimental evidence, the formation of colloidal Pd-containing nanoparticles was observed in situ for different Pd/NHC complexes in the studied reaction mixtures.